Hospital beds have evolved into complicated assemblies that, in many instances, provide therapeutic relief to patients that use them. A typical hospital bed includes a base designed to roll on the floor and a litter upon which the patient rests. A lift assembly, which often includes one or two jacks, connects the litter to the base so that litter can be raised or lowered as may be required for the convenience of the patient, the patient's medical condition, or to allow a specific medical procedure to be performed on the patient. Many hospital bed lift assemblies are also designed to selectively move one end of the litter so that either the head end or foot end of the litter can be elevated relative to the other end so as to incline the litter. The ability a modern hospital bed has to lift a patient up and down and to position the patient in an inclined position has been found to help individuals suffering from, shock, certain cardiac conditions and other medical ailments.
Many hospital beds are designed so that the lift assembly includes two jacks each of which extends between a separate end of the base and the adjacent end of the litter. Typically a hospital bed jack has a base unit that is secured to the bed base and a telescoping piston arm that extends between the base unit and the underside of the litter. Many of these jacks are hydraulically driven. The drive units used to actuate these jacks are arranged to raise the jacks in unison and further designed to individually lower them so that the litter unit can be inclined as desired. On many hospital beds the jacks are spaced as widely apart as possible in order to facilitate the placement of medical equipment below the litter so that it can be located underneath the patient. For example, it is desirable to have a space free below the patient in order to facilitate the positioning of X-ray receiving equipment underneath the patient.
While present hospital beds have proved to be useful devices that can ease a patient's hospital stay and offer some therapeutic relief, they are not without some disadvantages. When the telescoping jack piston arms are extended, they have a tendency to wobble around the base units to which they are attached. This jack wobble reduces the stability of the litter. Consequently, when uneven loads are applied to the litter, as can happen when the patient moves to one side of the litter or another individual presses down on a side of the litter, the litter may rock from side-to-side. Litters have also been known to rock when a hospital bed is used as a stretcher to move a patient from one location to another. In these situations, the movement of the bed sometimes causes the litter to develop a back-and-forth harmonic rocking motion. Whatever the cause, this rocking motion has been found to be disconcerting to patients.
Moreover, when a jack piston arm wobbles, it sometimes becomes stuck to the adjacent surfaces of the jack base unit in which it is housed. This situation typically occurs when the piston arm is fully extended and its range of side-to-side movement is at a maximum. Once the piston arm becomes stuck, it may not automatically retract when the jack is lowered. When this occurs, medical personnel have to spend time shifting the litter in order to free the piston arm so that the arm will return to its retracted position. Patients lying in their beds while the jacks are being freed have found this activity to be disquieting.
A number of solutions have been tried to minimize jack wobble and the disconcerting litter rocking that it causes. There have been attempts to cure this problem by providing jacks with relatively large diameter piston arms. While there is less rocking associated with these arms, they are quite costly to install. Moreover, these relatively large jacks significantly add to the overall weight of the hospital bed which, in turn, has made moving these beds more difficult. Another disadvantage of these assemblies is that the larger-sized jacks form larger obstructions that restrict the ability to place medical equipment below the litter underneath the patient.
Another solution to the problem of litter rocking has been to assemble the jacks from components that have very tight tolerances. By so assembling the jacks, it has been found that the piston arms move less and the litter rocking is dampened. Again though, it has however proved very costly to provide these components. Moreover, over time, the components forming these jacks start to wear. Once this occurs, the components lose their tight tolerances relative to each other and the jack piston arms start to develop a noticeable wobble.
Still another method that has been employed to reduce litter rocking has been to provide jacks with telescoping members that have clearances that can be selectively screw adjusted relative to each other. Once these members are set, there is a reduction in litter rocking. However, over time, the screw adjustment components become worn and their litter stabilizing capabilities decrease. Consequently, it becomes necessary to reset the screw adjustment components so that the telescoping members maintain the desired clearances relative to each other. The need to have to perform these readjustments adds to the workload of hospital maintenance personnel. Moreover, eventually, the screw adjustment components of these jack assemblies completely wear out. Once this occurs, the ability of these jacks to inhibit litter rocking is significantly reduced. Also, as before, it has proven expensive to provide the components required to assemble these jacks.